Here is the sentence including the word.
ふざけんじゃねえよ パンチに理{り}屈{くつ}なんざ無{む}用{よう}だい!
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「なんざ」＝「なんぞ + は」, originally.
「～～なんぞは」⇒「～～なんざあ」⇒「～～なんざ」
The last two are heard almost exclusively around Kanto.
Meaning: Same as 「なんかは」 or 「などは」. ("Stuff like ~~", "~~ and such", etc. )
answered Jul 14 '15 at 12:49
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"なんざ" is a colloquial form of "など" + "は".
Thus I would translate
パンチに理屈なんざ無用だい
into
No reasoning/justification is necessary for punches/fist-fighting.
