The phrase in query operates primarily as a suggestive idiom, usually used humorously or flirtatiously. It implies a playful supply of a “trip” (sometimes a kiss or different type of bodily affection) in change for, or because of, the presence of a mustache on the male particular person.
The enchantment, if any, derives from the underlying humor, the implied flirtation, and the cultural associations of mustaches themselves. All through historical past, mustaches have carried connotations starting from virility and class to rebelliousness and inventive aptitude. The phrase leverages these established associations to create a lighthearted, albeit doubtlessly suggestive, interplay. The “free” part provides a component of perceived worth or reward, additional incentivizing the interplay (in jest, in fact).
