Posted by Ian Horwill on October 19, 2010 at 14:20
In the article on "Mind your Ps and Qs", it seems to me that the original (1600s) quotes are talking about being in one's prime, which one can imagine morphing into the 1785 reference, as "to look for one's best opportunity". None of the possible origins mentioned in the article seem to fit well with this interpretation. A better fit is the "Prime Quality" theory that turns up if you search this site. The pluralization and introduction of "and" seem a bit gratuitous though.