Know how to use "fewer" and "less"? Find out.
late 15c., originally of feelings, from Middle French mutuel (14c.), from Latin mutuus "reciprocal, done in exchange," from PIE root *mei- "to change, exchange" (see mutable).
The essence of its meaning is that it involves the relation x is or does to y as y to x; & not the relation, x is or does to z as y to z. [Fowler]Mutual Admiration Society (1851) seems to have been coined by Thoreau. Mutual fund is recorded from 1950. The Cold War's mutual assured destruction attested from 1966. (Assured destruction was an early 1960s term in U.S. military policy circles in reference to nuclear weapons as a deterrent, popularized c.1964 by Robert McNamara, U.S. Secretary of Defense under Lyndon Johnson, e.g. statement before House Armed Services Committee, Feb. 18, 1965; the mutual perhaps first added by Donald Brennan, conservative defense analyst and a public critic of the policy, who also noted the acronym MAD.)
short for mutual fund, 1971; see mutual.