"dont something illness" is the kind of feeling i get from that.. cant find a verb to match to namena though.
In short, the phrase says: "Don't underestimate a disease!".
「なめんな」 is the informal spoken word for 「なめるな」, the negate form of 「なめる」, which has various meanings. The intended meaning in the said phrase is the last entry of meaning:
- to make fun of; to make light of; to put down; to treat with contempt; to underestimate
The informal spoken words are usually not found in physical dictionaries however, words that are closest to actual word may be suggested by online dictionaries.
So you would have to trace by the written word for it.
Not sure what was the original context in question. I can only guess there was someone who have said, "To fall ill is no big deal". Then the person whom had a cold (contracted a disease) replied, "Don't underestimate a disease [as illness could get worse if not taken into care]".