迫 (pressed/urgent) + 不及 (cannot reach) + 待 (to wait) - so urgent you cannot even manage to wait. Picture someone so excited they're already running before the starting whistle.
grammar
迫不及待 is used as an adverb before a verb, often with 地: 迫不及待地 + verb. Can also be used as a predicate: 我已经迫不及待了。
