Here is the sentence.
はったりにせよ おれたちをこいつらよばわりするたあ りっぱな度胸だぜ。
Is it a particle for emphasizing the sentence or making the verb よばわりする as imperative or something else?
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It is a sound change of "とは". It may be used in old city(下町 in Japanese) in Tokyo.
“はったりにせよ おれたちをこいつらよばわりするたあ りっぱな度胸だぜ” can be translated as “Even if it’s only a bluff, you have a good nerve to call us ‘Hey, cubs!’”
“たあ” is a colloquial and contracted form of “ …とは”, meaning “… is,” which is called “べらんめえ口調、” a local dialect in downtown areas of Tokyo and its environs.
“たあ” is always suffixed to verbs such as “する,” “言う,” “聞く,” in such a way as “ …するたあ、” “(なんて)言うたあ.” “….聞くたあ（驚いた）,” followed by a condemnation:
I cannot tolerate you feigning innocence at this stage.
You're a nut to have peeped in the women's section of a public bath.
or a surprise, like:
I'm surprised and greatly obliged to have our master's visit by himself to such a shabby house.
はったり…slang meaning of bravado for fight. a bluff.
The inflections of Edo dialect is as follows.
ai → ee（ない→ねえ、おまえ→おめえ）
oa → aa（とは→たあ）
ea → ya（それは→そりゃ）
There are two spoken languages in Edo(Tokyo).
One is 山手言葉(Yamate dialect) in west side, the other is 江戸言葉(Edo dialect) in east side.
Originally, Tokugawa shogunate family and their subordinates lived in west side.
In the period of the Meiji restoration, they leave and new government's people came there.
Therefore it is said that 江戸言葉 remained at the area of only east side.
It seems that the standard language of Japanese was made from 山手言葉.