loosen up is a verb meaning "cause to become unblocked". It carries 5 recorded senses.
become less tense, less formal, or less restrained, and assume a friendlier manner
“our new colleague relaxed when he saw that we were a friendly group”
become less tense, rest, or take one's ease
“He relaxed in the hot tub”
“Let's all relax after a hard day's work”
Definitions, synonyms and semantic relations for loosen up come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.