over one's skis

Etymology
Per Noreen Malone’s article in the Intelligencer: “the phrase’s metaphorical use seems to have begun in the finance world. Its first non-skiing print usages came in the early nineties, in publications like Investment Dealers’ Digest. For instance, a 1991 article quoted a “market source” describing a race between Goldman Sachs’ and Lehman Bros.’ preferred stock desk. “Someone said [Lehman] couldn’t get a clean legal opinion on it. They may have been out over their skis a little bit with their structure.””

https://nymag.com/intelligencer/2012/05/where-did-the-phrase-over-his-skis-come-from.html

As in skiing, the person in question has taken a position from which they must quickly back away. Taking a position that one lacks the ability to maintain. Quite normal in skiing, but costly in other fields.

Prepositional phrase

 * 1)  Acting or speaking prematurely; getting ahead of oneself.