Contracting 〜ているのだ to 〜てんだ
Grammar: Colloquial & Casual Speech
A verb ending in 〜て+いる followed by the particle のだ may contract as てんだ.
First, the い in 〜て+いる is dropped. Next, る becomes ん due to の following it. Finally, の becomes ん when followed by だ to です.