A sponger is someone who lives off other people — takes their money, their food, their spare room — and gives nothing back, and could work but doesn't. The word is an insult. You are not describing a situation; you are judging a person, and saying they are lazy and a bit shameless about it. Two things have to be true before people use it. First, the taking has to be repeated — one borrowed lunch is not spongeing, but the friend who has eaten at your table every week for a year and never once paid might get called one. Second, the person has to be seen as able to manage on their own. Nobody calls a small child or a sick person a sponger; they call the healthy cousin on the sofa one. It is British and rather informal — you would hear it in a pub or a family argument, not in a report. Americans more often say mooch or freeloader. Be careful with it in public talk: in Britain the word is often thrown at people receiving government money by those who think they should be working, so it can carry a political sting the speaker may not have meant to lend it.

吃白食的人、揩油的人、白吃白喝的傢伙；帶明顯貶意。台灣說的「靠別人養」「賴著不走的那種人」大致是這個意思