Learn Polish: To mięso śmierdzi podejrzanie. - This meat smells suspicious.

In Polish, sensory verbs like śmierdzieć (to stink), wyglądać (to look), and brzmieć (to sound) are always modified by adverbs. While English uses adjectives for this (it smells suspicious), Polish literally says that it smells suspiciously.

Does śmierdzieć just mean to smell, or does it always mean a bad smell?

It specifically means to stink or to smell bad. If you want to say something smells good, you use the verb pachnieć. If you want a neutral word for the physical act of smelling something with your nose, you use the verb wąchać.

What if I wanted to say this meat smells LIKE something suspicious, using a noun?

Sensory verbs in Polish take the instrumental case when followed by a noun. If you wanted to say it smells like a suspicious animal, you would say To mięso śmierdzi podejrzanym zwierzęciem. However, because our sentence just describes the quality of the smell without a noun, it requires the adverb podejrzanie.

Can I use To mięso pachnie podejrzanie to mean the exact same thing?

Grammatically it works, but the meaning shifts. Because pachnieć implies a pleasant fragrance, saying it smells suspiciously good or has a suspicious fragrance sounds ironic or contradictory. For describing meat that has likely gone bad, śmierdzieć is the correct verb.