12 November 2015
Leech is a noun with two distinct meanings, or perhaps it is more accurately stated that leech is two separate nouns that are spelled the same and often conflated. The word is the name for a type of blood-sucking invertebrate, and it also is an archaic term for a physician or healer. The two senses are associated because physicians used to (and in some limited applications still do) use the worms to draw blood out of a patient. The conflation of the two goes back to Old English, where læce is the word for both a physician and a bloodsucker, but the two senses apparently come from distinct roots.
The physician sense comes a root that corresponds to the Old Frisian letza and the Old Norse læknir. It comes from the proto-Indo-European root leg-, which has a sense of speech. Presumably physicians spoke magic, healing words. This PIE root would also give us words like lecture, legend, and intelligent.
The bloodsucker sense of læce, spelled lyce in the Kentish dialect of Old English, corresponds to the Middle Dutch lake or lieke, and has various Low German cognates. It stems from the PIE root leig-, which has a sense of tying or binding, and the worm probably gets its name from its shape, which resembles a rope. Other words from this PIE root include ligature, league, and religion.
Sources:
American Heritage Dictionary of Indo-European Roots, third edition, 2011, s. v., leg-1, leig-1
Oxford English Dictionary Online, second edition, 1989, s. v., leech, n.1, leech, n.2