Short utterance, gesture, or pause immediately after saying something that is intended to be funny, but usually isn't. The statement is usually practiced, but a swid means the speaker has nothing better to say and is temporarily speechless. The spelling is a backronym of "See What I Did?".
Everyone in the boardroom stared at the ceiling as his swid dropped to a whimper after yelling "that's what she said!" during the boss's presentation.