crossbones (n.)

word-forming element typically representing cross as a noun, adverb (cross-examine), adjective (crossbar), and in many words a confluence of them. "There is no distinct line of division between cross as an adjective and cross as a prefix. As a prefix, it often represents the adv. cross, or the prep. cross, across." [Century Dictionary]

Middle English bon, from Old English ban "bone, tusk, hard animal tissue forming the substance of the skeleton; one of the parts which make up the skeleton," from Proto-Germanic *bainan (source also of Old Frisian and Old Saxon ben, Old Norse bein, Danish ben, German Bein). Absent in Gothic, with no cognates outside Germanic (the common PIE root is *ost-); the Norse, Dutch, and German cognates also mean "shank of the leg," and this is the main meaning in Modern German, but English seems never to have had this sense.

To work (one's) fingers to the bone is from 1809. To have a bone to pick (1560s) is an image of a dog struggling to crack or gnaw a bone (to pick a bone "strip a bone by picking or gnawing" is attested from late 15c.); to be a bone of contention (1560s) is of two dogs fighting over a bone; the images seem to have become somewhat merged. Also compare bones.

Bone-china, which is mixed with bone-dust, is so called by 1854. Bone-shaker (1874) was an old name for the early type of bicycle, before rubber tires.