In
grammar, a form that is of a different origin from what you would expect to be a
related form. For example, in English the past tense of
go is not *"goed" (though archaic words
yode and
yede do exist) but
went, originally from the verb
wend.
In Latin fero 'I carry' would be expected to have perfect something like *ferui and supine *fertum, but in fact it uses tuli and latum. (These two themselves look unlike but latum comes from earlier tlatum, the same root as in Greek Atlas, the bearer.)