From WordNet (r) 3.0 (2006) [wn]:
tense up
v 1: become tense, nervous, or uneasy; "He tensed up when he saw
his opponent enter the room" [syn: tense, tense up]
[ant: decompress, loosen up, relax, slow down,
unbend, unwind]
2: cause to be tense and uneasy or nervous or anxious; "he got a
phone call from his lawyer that tensed him up" [syn: tense,
strain, tense up] [ant: loosen up, make relaxed,
relax, unlax, unstrain, unwind]