Mot juste is French for "the right word for the right time" (mot: word, juste: right). The correct word for the situation.
"The Lord God hath given me the tongue of the learned, that I should know how to speak a word in season to him that is weary (tired? sounds about right): he wakeneth morning by morning, he wakeneth mine ear to hear as the learned." This is speaking of Jesus. There's no reason, however that it can't be spoken of us, too.
Here's an example of, not just a specific word, but also the right situation in which said word (phrase, whatever) applies. Real quick, if there's anyone who knows just what to say at the right time, it's Jesus. Think about the time He sat on the shore of Tiberias and talked to Peter. Peter had denied Him however many nights before and must've felt quite the failure. Maybe that's one of the reasons he went back to his old station as fisherman. This new thing that Jesus began by calling out random and varied individuals from society to follow Him, seems to have failed. A flash in the pan. So, Peter went back to what he did best. "I go a fishing" (John 21:3) And so then Jesus stands on the shore and calls out to Peter. History seems to be repeating itself. In much the same way did Jesus originally call Peter. Only it was on the shore of Galilee and Peter was already there on the shore with his brother Andrew, fixing their fishing net (Matthew 4:18). Think of all the things that had transpired in the three years since they first met. And now they find themselves together again. Jesus is victorious, having triumphed over all the hordes and hosts of hell after dying on the cross. It's that very thing that Peter must've stumbled over. Remember the previous chapter? Peter and John (the "beloved" disciple) both go to the tomb on the first day of the week. John, it says, "believed" (20:8). The next verse says "For as yet they knew not the scripture, that He must rise from the dead." It continues, not mentioning the (assumed) fact that Peter doesn't realize that Jesus rose from the dead: "Then the disciples went away again unto their own home." So fast-forward to that morning, however many days later and see Jesus call out to Peter on the lake of Tiberias. If you'll remember, it was John ("that disciple whom Jesus loved" 21:7) who recognized Him first. As an aside, who knows why they didn't initially know who it was Him (vs. 4)? It could have been dark. Foggy maybe. Jesus may well have taken on a different form after He rose. Who knows? God does. It wasn't the first time He wasn't recognized by those who knew Him prior to His death. It speaks to the inward witness of the Holy Spirit that lets us know who Jesus really is. I'm digressing. So Peter hears Jesus call ("Children, have ye any meat?" 21:5) and upon realizing just who it is, flings himself overboard and swims ashore. I say all of that to say this: consider all of these details and the mindset in Peter they might create. Because when Jesus sits down with him opposite the campfire, He says (asks, really) the one. Pertinent. Question: "Lovest thou me?" The right word at the right time. Mot Juste. Notice. He's not angry with Peter. He doesn't chide him for abandoning the ministry. He doesn't even mention the fact that Peter denied Him three times the night He was crucified. Though I can imagine the look that Jesus gave him when he denied Him the third time (see Luke 22:61)... All He asks is if Peter loves Him. The corollaries of which span our entire lives. Do we love Jesus? Don't answer that without thinking.
And here's an example of the opposite.
"He that blesseth his friend with a loud voice, rising early in the morning, it shall be counted a curse to him." (Proverbs 27:14)
How many times do you wake up and the cares of this life threaten to inundate you? Another question: assuming that sort of thing happens with frequency, how many times do you then realize that the worries and cares and pressures are nothing that you can't handle (with God's help, of course)? For me, that's most days. It may have something to do with routine. It may have something to do with my brain chemistry and physiology, though that's, like, the lowest tier. It may indeed have something to do with the fact that the devil seeks to derail my day even before it's begun. I don't know. And after my head clears (with or without coffee), I don't even care. And notice what Solomon says here. "He that blesseth his friend..." In this case, it doesn't even matter what's said. It's the timing that's off. The example: Two friends. The one calls the other early in the morning (one's a morning person, one's not, you understand) and wants to hang and goof off. And the other friend is now dealing with the aforementioned minutae of torment as so many of us are wont to do. Explained thusly, I wouldn't fault the second friend for hanging up, and not hanging out. The word 'curse', as it's translated in the King James, connotes "disparagement" and "mistreatment". In other words, it could potentially throw a wrench in the friendship. Not good.
On a certain level, the verse from Proverbs may well be taken as the exact opposite of what Jesus did. There are finer points that render such a shallow interpretation invalid, but consider. Jesus, rising early, blessed Peter with a loud voice. Though it had the opposite effect in their case. It would seem a curse was lifted from Peter's shoulders there on the shore. Only Jesus could do something like that. He'll do it for us.
"For it is not ye that speak, but the Spirit of your Father which speaketh in you." (Matthew 10:20)