抽水 'pull water / pump water' + 马桶 'toilet' — the 'water-pumping toilet', i.e. the modern flushing kind, as opposed to a chamber pot. Plain 马桶 alone is also fine in everyday speech.
culture
马桶 originally meant a wooden chamber pot — folk etymology says 桶 'bucket' was prefaced with 马 because the bucket was as broad as a horse's belly. With plumbing, 抽水马桶 became the proper term in technical/sales contexts; everyday speech keeps it short.
