verb/ku.pe l‿ɛʁ.b(ə) su lə pje/
to beat to the punch, cut the ground from under someone's feet
“J'allais le dire, mais tu m'as coupé l'herbe sous le pied.” — I was about to say it, but you beat me to it.
Synonyms: devancer
Literally, “to cut the grass from under the foot”.