It’s the standard transcription of the Arabic sound, which is closest (in fact, pretty much identical) to the Hebrew sound which is typically used to pronounce the first consonant in Hamas. Blame the bad Arabic-language education is Israel for not pronouncing it correctly…
This is, as far as I know, not a word in Arabic, and non-standard transcription for Hebrew — the standard would be “Ḥamas” or “H̲amas” (using “ch” is common but non-standard as well).