sand-trap (n.)

"water-worn detritus finer than gravel; fine particles of rocks (largely crystalline rocks, especially quartz); the material of the beach, desert, or sea-bed;" Old English sand, from Proto-Germanic *sandam (source also of Old Norse sandr, Old Frisian sond, Middle Dutch sant, Dutch zand, German Sand), from PIE *bhs-amadho- (source also of Greek psammos "sand;" Latin sabulum "coarse sand," which is the source of Italian sabbia, French sable), suffixed form of root *bhes- "to rub."

Historically, the line between sand and gravel cannot be distinctly drawn. Used figuratively in Old English in reference to innumerability and instability. General Germanic, but not attested in Gothic, which used in this sense malma, related to Old High German melm "dust," the first element of the Swedish city name Malmö (the second element meaning "island"), and to Latin molere "to grind."

Metaphoric for innumerability since Old English. In compounds, often indicating "of the shore, found on sandy beaches." In old U.S. colloquial use, "grit, endurance, pluck" (1867), especially in have sand in (one's) craw. Sands "tract or region composed of sand," is by mid-15c.

"contrivance for catching unawares," late Old English træppe, treppe "snare, trap," from Proto-Germanic *trep- (source also of Middle Dutch trappe "trap, snare"), related to Germanic words for "stair, step, tread" (Middle Dutch, Middle Low German trappe, treppe, German Treppe "step, stair," English tread (v.)).

This is probably literally "that on or into which one steps," from PIE *dreb-, an extended form of a root *der- (1), base of words meaning "to run, walk, step." The English word is probably akin to Old French trape, Spanish trampa "trap, pit, snare," but the exact relationship is uncertain.

The sense of "deceitful practice, device or contrivance to betray one" is recorded from c. 1400. The meaning "U-shaped section of a drain pipe" is from 1833. Slang meaning "mouth" is from 1776. Speed trap is by 1908. Trap-door "door in a floor or ceiling" (often hidden and leading to a passageway or secret place) is attested from late 14c. (trappe-dore).