Hi... Meaning : Dishonest or treacherous behaviour; also violent conduct. Origin : 'Foul play' is a 16th century idiom. Nowadays we often use this phrase in regard to 'fouls' that are committed in sports, i.e. actions which are outside the particular sports' rules. This is itself quite an old usage. For example, from boxing - The Sporting Magazine, 1797: "His antagonist having struck him two foul blows." ... and from billiards - The Field, January 1882: "Thus, at billiards, if a player makes a foul stroke and scores, his adversary has the option of not enforcing the penalty." These were preceded by Shakespeare's use, and probably his coinage, of the phrase in a non-sporting context, simply to mean 'unfair behaviour'. For example, Love's Labours Lost, 1588: BIRON: Honest plain words best pierce the ear of grief; And by these badges understand the king. For your fair sakes have we neglected time, Play'd foul play with our oaths. Shakespeare used this phrase in several plays, including Henry IV, The Tempest and Pericles, Prince of Tyre.

it could be Unfair or dishonest behaviour (especially involving violence) cowboy...

Foul play was not suspected and authorities did not know how the two planned to get to San Francisco. Hi, here 'foul play 'means that the police did not think that anything bad had happened.

Few extra examples: * The boy accused his friend of foul play (= underhanded behavior) when he started dating his girlfriend in his absense. * The current Administration said they won the election fair and square, but most observers suspected foul play (= like election fraud). * They could never find a body, but the way she suddenly disappeared, most people suspected foul play (= her having become the victim of a crime). The last sentence carries the same meaning as in your story.

it can mean a few things, yes like mark said it can mean illegal..it can also be used casually to say someone did something they shouldnt or that was underhand etc. also there is the litteral foul play in football lol!