loosen (v.) Look up loosen at Dictionary.com
late 14c., losnen (transitive) "make loose, free from tightness," later lousen (early 15c.), from loose (v.) + -en (1). Intransitive sense of "become loose" is from 1670s. Meaning "limber the muscles before physical effort" is from 1955. Related: Loosened; loosening; loosener.