not your long suit

not (one's) long suit

Not something one is particularly adept at. I'd avoid going for humor in your speech—jokes are not your long suit. Social etiquette is definitely not my long suit.
See also: long, not, suit
Farlex Dictionary of Idioms.

not your long suit

If something is not your long suit, you are not good at it. Note: The following expressions refer to the four suits in a pack of cards: diamonds, hearts, clubs, and spades. Tact has never been my long suit. Charm was never Michael's long suit. Note: If a large number of a player's cards belong to a particular suit, you can call that suit their long suit.
See also: long, not, suit
Collins COBUILD Idioms Dictionary, 3rd ed.
See also:
  • not (one's) long suit
  • definitely
  • definitely not
  • Definitely not!
  • hope not
  • (I) hope not
  • not for a second
  • not for a minute
  • not for a moment
  • ain't