col legno
Concise Oxford English Dictionary © 2008 Oxford University Press:
col legno /kɒl ˈlɛnjəʊ/
▶adverb Music (with reference to a bowed instrument) played by hitting the strings with the back of the bow.
– origin Ital., ‘with the wood (of the bow)’.
'col legno' also found in these Oxford entries:

