A male donkey.
"The farmer gently guided the stubborn jackass to the water trough."
to behave very obnoxiously
"The guy at the front of the line kept honking his horn and making loud noises, acting like a total jackass."
a jack and an ace as a starting hand in Texas hold 'em due to phonetic similarity
"After calling for a pair of jacks, I was stunned when my opponent shoved all-in on the river, only to reveal the perfect jack-ace combination that had been sitting there since the flop."