へそで茶(ちゃ)を沸(わ)かす (heso de cha o wakasu) literally means ‘I’d make tea using my navel’. If you say this phrase, it figuratively means that you think what someone has said is ludicrous and unrealistic, like making tea using one’s navel.
It’s usually used as “へそで茶を沸かすわ” when you hear someone saying something ludicrous and scorn and make fun of them.
Do you aim to become a professional soccer player from now? Hah, I’d make tea using my navel! (That’s so funny/ludicrous!)
噴飯（ふんぱん）もの: “so ludicrous that you spit out drink/food when you hear it”
非現実的 (ひげんじつてき): “unrealistic”
絵(え)に描(か)いた餅(もち): “pie in the sky”