Don’t Feel Badly; Instead, Feel Bad
Bad is an adjective that is used with linking verbs (verbs of being, such as be, become, seem, feel, taste, look, smell).

Don’t feel bad about slamming my hand in the car door; it happens all the time.
I feel bad about dropping you off in the middle of the highway, but I’m running really late.
He looks bad; it must be the flu.
This soup tastes bad; it reminds me of my grandmother’s cooking.
Badly is an adverb, and it describes how you do something (or, as some would say, it modifies a verb–as long as it isn’t a linking verb). She sings badly means that you cover your ears whenever she breaks into her operatic rendition of I Will Survive. He swings the bat badly could mean that he swings it so erratically that he has to yell heads up! every time he steps up to the plate.

To feel badly means that your sense of touch is, as the phrase goes, out of wack. This tastes badly means that the food you’re talking about is actually what’s doing the tasting, and it’s somehow doing it in a bad way. Talk about bad taste! And Dannyboy, measurable, you are correct, one word just saying, la la la.......