armchair

an armchair critic a person who knows about a subject only by reading or hearing about it and criticizes without active experience or first-hand knowledge.

☞ The phrase armchair critic is first recorded in 1896, but the concept was around at least a decade earlier: in 1886 Joseph Chamberlain sneered at opponents as 'arm-chair politicians'. Another common variation on the theme is armchair traveller, meaning 'someone who travels in their imagination only'.