Learn English – Is “grossingly” a word

In standard English usage, fellow is neutral and informal, as Jim notes in his comment. It would be appropriate for relaxed gatherings. Think of a speaker at a technology conference, for instance:

"And I tell you, this fellow right here, he is an inspiration to all of us."

It would also be appropriate at parties. Think of the song:

"For he's a jolly good fellow."

On the other hand, it can be used negatively as appropriate. In these contexts it would indicate derision or intensity.

"Nasty fellow, always coming up with some new scheme for getting his own money with other peoples' effort."

"I won't deny, he works like a horse, but nice? No, he's definitely not a nice fellow."

The use of the word should be a deliberate choice if the context is not completely casual. In a small group of friends having drinks together, it's pretty much always appropriate. A speaker might use it to help loosen up his audience, as in the first sentence I noted. Using it in writing is inadvisable unless the context specfically calls for informality and a folksy feel. A find/replace from person to fellow is not a safe or good idea.

OED tells us that it is an adverb with the meaning of "half", but that it is obsolete:

Obs. = half adv.

^{c1480 (▸a1400) St. Ninian 1418 in W. M. Metcalfe Legends Saints Sc. Dial. (1896) II. 344 Til hyme, þat halfly-slepand lay.
1565 J. Hall Hist. Expost. in tr. Lanfranc Most Excellent Woorke Chirurg. sig. Dddv, Thine arte is halflye wunne.
1622 M. Drayton 2nd Pt. Poly-olbion xxiv. 83 So holy that him there, they halfely deifide.
1674 N. Fairfax Treat. Bulk & Selvedge 167 This is what it is halfly.}

The most recent citation is from 1674, but it has happened before that words thought to be obsolete are revived or evolve anew from familiar forms. If singly and on(e)ly are adverbs, it is no stretch to grant halfly the same label.