Old English heap "pile (of things); great number, crowd, multitude (of persons)," from West Germanic *haupaz (source also of Old Saxon hop, Old Frisian hap, Middle Low German hupe, Dutch hoop, German Haufe "heap"), of uncertain origin. The group is perhaps related to Old English heah "high" (see high), but OED suggests a common origin with Latin cubare "lie down," and Boutkan says it is probably not Indo-European at all.
Slang meaning "old car" is attested from 1924. Earlier it meant "slovenly woman" (1806). As a characteristic word in American Indian English speech, "a lot, a great deal," by 1832.
One grain of sand does not make a heap. A second grain of sand added to the first does not make a heap. Indeed each and every grain of sand, when added to the others, does not make a heap which was not a heap before. Therefore, all the grains of sand in existence can still not a heap make. [the fallacy of the heap, as described in Malcolm Murray and Nebojsa Kujundzic, "Critical Reflection," 2005]