expression, noun-modifying (irregular)
outspoken; forthright; frank; direct
Idiomatic expression meaning to speak one's mind without mincing words. Literally 'not putting clothes on one's teeth', implying bluntness. Often used as a prenominal modifier.
彼は歯に衣着せぬ物言いで知られている。
He is known for his outspoken manner of speaking.
歯に衣着せぬ意見を聞かせてください。
Please give me your frank opinion.
遠慮なく means 'without reserve' or 'without hesitation', but 歯に衣着せぬ specifically conveys a sharp, unfiltered directness in speech.
From the literal image of not putting clothes (衣) on one's teeth (歯), meaning to speak without covering or softening one's words. The exact origin is uncertain, but it is a well-established idiomatic expression.