the compound "straight-up" is a modern american english formation that combines two ancient germanic elements. the word "straight" evolved from the old english concept of being stretched or extended, which came to signify directness and lack of deviation. the adverb "up" adds a sense of completeness or totality, a usage that developed from its spatial meaning. when combined, the literal sense of "directly upward" was metaphorically extended to describe a person's forthright character. in bartending, the term further specialized to mean a liquor served in its pure, unadulterated state, aligning with the core idea of something being direct and undiluted.

Structure: straight (direct) + up (completely) straight (from middle english streght, from old english streht, past participle of streccan 'to stretch', of germanic origin; functional role: adjective root meaning direct, not curved). up (from old english up, upp, of germanic origin; functional role: adverb particle intensifying the meaning of the adjective).