"back of the foot," Old English hela, from Proto-Germanic *hanhilon (source also of Old Norse hæll, Old Frisian hel, Dutch hiel), from PIE *kenk- (3) "heel, bend of the knee" (source also of Old English hoh "hock").
Meaning "back of a shoe or boot" is c. 1400. Down at heels (1732) refers to heels of boots or shoes worn down and the owner too poor to replace them. For Achilles' heel "only vulnerable spot" see Achilles. To fight with (one's) heels (fighten with heles) in Middle English meant "to run away."
"contemptible person," 1914 in U.S. underworld slang, originally "incompetent or worthless criminal," perhaps from a sense of "person in the lowest position" and thus from heel (n.1).
"to lean to one side," usually in reference to a ship, re-spelled 16c. from Middle English hield (probably by misinterpretation of -d as a past tense suffix), from Old English hieldan "incline, lean, slope," from Proto-Germanic *helthijan (source also of Middle Dutch helden "to lean," Dutch hellen, Old Norse hallr "inclined," Old High German halda, German halde "slope, declivity"). Related: Heeled; heeling.
"furnish with a heel," of a shoe, boot, etc., c.1600, from heel (n.1). Related: Heeled; heeling.