From an element of uncertain interpretation pes (which is perhaps used in its "post-verbal intensifier" sense) + tsawg (“few”).
pes tsawg
- how much? how many?
- Koj yuav pes tsawg tus? ― How many do you want?
- Heimbach, Ernest E. (1979) White Hmong — English Dictionary[1], SEAP Publications, →ISBN, page 228.