Used before a noun to mark something as only *claimed* — other people say it is so, and you are quietly signalling that you don't buy it. "The supposed expert couldn't answer a single question." "Where is this supposed improvement?" The word does not describe the thing; it describes the claim about the thing. A supposed thief may be entirely innocent — you are reporting an accusation, not making one. Because of this, journalists and lawyers reach for it (along with *alleged*) when they must repeat a claim without endorsing it, and ordinary speakers reach for it when they are being sarcastic: say "your supposed friend" and everyone hears the eye-roll. Note that this adjective is normally said with the -ed pronounced as a full syllable, /səˈpəʊzɪd/, and it sits *before* the noun; you would not say "the expert was supposed." This is a different animal from the very common *be supposed to* (= should, or = people say), which A1-A2 students meet first — "I'm supposed to be home by ten." Same spelling, different job.