Couldn't quite understand the meaning of word "clobber". "hit (someone) hard." or "treat or deal with harshly." gcc manual 6.45.2.6 Clobbers
"Clobber" in this context is common jargon for "overwrite arbitrarily". In other words, declaring a register as "clobbered" tells the compiler that this register may be overwritten with some other arbitrary value, and that the compiler should not assume anything about its contents when the asm code completes; its previous value is considered lost.
It's loosely from the meaning "hit": the analogy is that overwriting a register is like damaging or destroying its previous value, as if with physical force.
The manual defines it further down:
Clobbers: A comma-separated list of registers or other values changed by the AssemblerTemplate [...]