Green’s Dictionary of Slang

(US) to be alert, to watch out.

1970		Current Sl. V:3 13: Truchas! exclam. Look out!
1993		L. Rodríguez Always Running (1996) 88: Trucha, homes .
1997		Lil’ Rob ‘Jump in the Ride’ 🎵 Crazy Life [album] Yeah, it’s time for jacking, my homeboys are-a packing / Your fucking carrucha got the fusca so keep trucha / Cuz if you got what we want / If it’s firme as they say and it bumps in your trunk / Then hey man that shit is getting taken.
1999		E. Bunker Mr Blue 107: Once I accidentally ran a stop light on Riverside Drive. She said, ‘Trucha . . . la jura!’ It was pure barrio slang for ‘Cool it, the heat.’.