This is again a mash up of “one hand washing the other” and “you scratch my back; I’ll scratch yours” (see yesterday’s malaphor post). These two malaphors were both spoken by “the Master”, so why would he mix them both in different ways? My guess is that he may have been thinking in this one of “watching your back” (looking out for another) since “watch” and “wash” are similar sounding words (“the master” did tend to slur words, especially after a long lunch). Again, it is difficult to step into such a complex mind so I can only speculate.
This is a mix up of two similar meaning idioms – “one hand washing the other” and ” you scratch my back, I’ll scratch yours” (reciprocal help). These malaphors that mix idioms having the same or similar meaning are also called congruent conflations.