The Collaborative International Dictionary of English v.0.48:
Unstrain \Un*strain"\, v. t. [1st pref. un- + strain.] To relieve from a strain; to relax. --B. Jonson. [1913 Webster]WordNet (r) 3.0 (2006):
unstrain v 1: cause to feel relaxed; "A hot bath always relaxes me" [syn: relax, unstrain, unlax, loosen up, unwind, make relaxed] [ant: strain, tense, tense up]