throw in at the deep end

From allusion to an actual swimming pool.

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throw in at the deep end (third-person singular simple present throws in at the deep end, present participle throwing in at the deep end, simple past threw in at the deep end, past participle thrown in at the deep end)

1. (idiomatic) to introduce a person to a new situation without adequately preparing him or her.

• hit the ground running
• sink or swim
• in at the deep end