The Collaborative International Dictionary
Prewarn
Prewarn \Pre*warn"\, v. t. & i. [imp. & p. p. Prewarned; p. pr. & vb. n. Prewarning.] To warn beforehand; to forewarn. [R.]
Wiktionary
prewarn
vb. (context transitive English) To warn beforehand; to forewarn.