Why was "tantrum" trending last week?
mid-15c., "to change, transform," from Latin commutare "to often change, to change altogether," from com-, intensive prefix (see com-), + mutare "to change" (see mutable). Sense of "make less severe" is 1630s. Sense of "go back and forth to work" is 1889, from commutation ticket "season pass" (on a railroad, streetcar line, etc.), from commute in its sense of "to change one kind of payment into another" (1795), especially "to combine a number of payments into a single one." Related: Commuted; commuting.