throw in at the deep end

throw sb in at the deep end (give sb a difficult job to do, or a serious problem to deal with, before they have the knowledge or experience for it; give no support or help) — дать трудную работу и предоставить самому с ней разобраться;
~ бросить в пучину, предоставив выплывать самостоятельно;
~ бросить на произвол судьбы (контекст.);

Example 1: She was just thrown in at the deep end with her new job. No-one helped her at all.

Example 2: When I first started they just threw me in at the deep end. Our boss thinks the best way to introduce new staff to the job is to throw them in at the deep end and see how they go.

Origin
Metaphorical. Probably related to the fact that swimming pools usually have a deep end and a shallow end . If someone is thrown in at the deep end , they'll drown unless they manage to get to the side of the pool.