foot fault 
noun
(In tennis, squash, and similar games) an infringement of the rules made by overstepping the baseline when serving.Hewitt serves his second double fault of the match and is then called for a foot fault to give Federer a set point....
- Williams admitted to being a little rusty, which showed in several double faults and foot faults.
- He famously mooned a referee, threw a shoe at a baseline judge who kept calling foot faults and changed both his shirt and his shorts on court during a match.
verb
1 (foot-fault) [no object] (Of a player) make a foot fault.
1.1 [with object] Award a foot fault against (a player).On his way to the US Open final he caused uproar when he yelled at a black linesmen who had foot-faulted him....
- Given this resort's status as a celebrity magnet, you may fear being foot-faulted if you're not wearing Jimmy Choos on court.