"small wheel or roller," late 14c., from Anglo-French trocle, from Latin trochlea "a small wheel, sheaf of a pulley," from Greek trokhileia "a system of pulleys," from trokhos "wheel," from trekhein "to run," from PIE root *dhregh- "to run" (source also of Old Irish droch "wheel," Lithuanian drožti "to run fast"). Truckle bed "small bed on wheels that can be stowed under a larger bed" is from mid-15c.
"give up or submit tamely," 1610s, originally "sleep in a truckle bed" (see truckle (n.)). Meaning "give precedence, assume a submissive position" (1650s, implied in truckling) is perhaps in reference to that type of bed being used by servants and inferiors or simply occupying the lower position. Related: Truckled; truckling.