According to the OED, the earliest attested use of "cheat on" in the sense "be sexually unfaithful to (one's spouse)" dates to 1934. The reference given is:

John Henry O'Hara · Appointment in Samarra · 1934 (UK 1935), ch.6/p.159

The OED reports that the phrase was chiefly used in US English, and this seems to be supported by subsequent citations; however, that says nothing about its geographic spread in recent years.

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