I. arm‧chair¹ /ˈɑːmtʃeə, ˌɑːmˈtʃeə $ ˈɑːrmtʃer, ˌɑːrmˈtʃer/ noun [countable]
a comfortable chair with sides that you can rest your arms on

[TahlilGaran] Dictionary of Contemporary English ▲

II. arm‧chair² /ˈɑːmtʃeə $ ˈɑːrmtʃer/ adjective
armchair traveller/fan etc someone who talks or reads about being a traveller, or watches sport on television but does not have any real experience of doing it:
Her books about her adventures give enjoyment and inspiration to armchair travellers.
Armchair fans will have to pay extra to watch the best games live.

[TahlilGaran] Dictionary of Contemporary English ▲

armchair
noun
ADJ. big, deep, large | comfortable, comfy | battered, old, worn | leather, upholstered, wicker, wooden
VERB + ARMCHAIR be seated in, be slumped in, be sprawled in, sit in | collapse into, flop into, throw yourself into | curl up in, ease yourself into, settle into He eased himself into the big armchair.

[TahlilGaran] Collocations Dictionary ▲

armchair
noun
BAD: She was sitting on her favourite armchair.
GOOD: She was sitting in her favourite armchair.

Usage Note:
sit (down) in an armchair (NOT on ): 'He sat down in the armchair and almost immediately fell asleep.'

[TahlilGaran] Dictionary of Common Errors ▲