What is the X in X-mas?
mid-14c., "narrow, confined space or place," specifically of bodies of water from late 14c., noun use of adjective strait "narrow, strict" (late 13c.), from Old French estreit (French étroit) "tight, close, narrow" (also used as a noun), from Latin strictus, past participle of stringere "bind or draw tight" (see strain (v.)).
Sense of "difficulty, plight" (usually straits) first recorded 1540s. Strait and narrow "conventional way of life" is recorded from mid-14c. (see straight (adj.2)).
A narrow passage, such as the upper or lower opening of the pelvic canal.