*fortu- — de Vaan
*fortu- 'chance, luck', whence an adj. *fortuno- 'lucky'. The precise origin of -Ttus in fortu-itus and in its semantic neighbour gratu-itus 'free of charge' is unclear. The semantic shift from 'load' or 'the carrying' to 'chance, luck' is not obvious, and EM go so far as to reject the connection between fors and few* Yet the co-occurrence of //- and /w-stems seems old, and there are many ways in which a meaning … — [de Vaan, s.v. *fortu-, p. 250]