Толковый словарь английского языка Oxford English Reference - warn

v.tr. 1 (also absol.) a (often foll. by of, or that + clause, or to + infin.) inform of danger, unknown circumstances, etc. (warned them of the danger; warned her that she was being watched; warned him to expect a visit). b (often foll. by against) inform (a person etc.) about a specific danger, hostile person, etc. (warned her against trusting him). 2 (usu. with neg.) admonish; tell forcefully (has been warned not to go). 3 give (a person) cautionary notice regarding conduct etc. (shall not warn you again). Phrases and idioms warn off 1 tell (a person) to keep away (from). 2 prohibit from attending races, esp. at a specified course. Derivatives warner n. Etymology: OE war(e)nian, wearnian ult. f. Gmc: cf. WARE(3)