telly vs idiot box

telly and idiot box both are nouns.

Word	Noun	Adjective	Verb	Adverb
telly	Yes	No	No	No
idiot box	Yes	No	No	No

As nouns, telly and idiot box are synonyms defined as:

telly and idiot box: an electronic device that receives television signals and displays them on a screen

Other synonyms of telly include boob tube, goggle box, television, television receiver, television set, tv, tv set.

telly (noun)	idiot box (noun)
an electronic device that receives television signals and displays them on a screen	an electronic device that receives television signals and displays them on a screen