Learn Polish: Szczerze mówiąc, nic o nim nie wiem. - Honestly speaking, I know nothing about him.

The word mówiąc is an adverbial participle, meaning 'speaking.' Fixed phrases like szczerze mówiąc (honestly speaking) use this -ąc form to express your attitude about the statement you are making. Unlike normal verbs, it doesn't change for person or tense, making it a reliable set phrase to frame your honest opinion.

Is the comma after szczerze mówiąc strictly necessary?

Yes, Polish punctuation rules require a comma here. Framing phrases and participles like szczerze mówiąc (honestly speaking) or ogólnie rzecz biorąc (generally speaking) must be separated from the main clause by a comma, very similar to introductory phrases in English.

I see nic and nie in the same sentence. Is this a standard double negative?

Exactly. Polish requires a double negative. Whenever you use a negative word like nic (nothing), nikt (nobody), or nigdy (never), the main verb must also be negated with nie. You literally say 'nothing about him I do not know.'

Why does the pronoun on (he) change to nim in this sentence?

The preposition o takes the Locative case when it means 'about' (as in knowing or talking about a topic). Nim is the Locative form of on. Notice that it starts with an 'n' because it directly follows a preposition, which is a common pattern for Polish pronouns.