an armchair critic

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 variant is armchair traveller , meaning ‘someone who travels in their imagination only’.
See also: armchair, critic