The noun derives from the Latin root pulcher meaning beauty. It is usually a reference to a shapely or voluptuous feminine form, and as a slang reference to an ample female bosom. The adjective form is pulchritudinous.
The phrase 'pulchritude incognita' contains an error. Instead, 'pulchritude' should be written 'pulchritudo', as the subject of the sentence, in the nominative case. The correct phrase therefore is...