"that which portends, an omen," generally a bad one, 1560s, from Middle French portente, from Latin portentum "a sign, token, omen; monster, monstrosity," noun use of neuter of portentus, past participle of portendere (see portend).
portal
portcullis
Porte
porte-cochere
portend
portent
portentous
porter
porter-house
portfolio
porthole