(Hakuhyou wo fumu ga gotoshi; “Like treading on thin ice”)
Being in an intensely dangerous situation. When a thick layer of ice covers a body of water, it can safely support people walking on it or even heavy loads of freight on a sledge. If the ice is thin, then it can break without warning and drop you into near-freezing water, which can easily be fatal. This saying compares a situation to that danger. Note, though, that this kotowaza does not refer to a dangerous venture undertaken knowingly; that usage is considered an error.
薄氷 (hakuhyou) is literally “thin ice.” This noun is marked by the direct-object particle を (wo) as being acted on by the verb 履む (fumu), which can take on a number of different meanings but here becomes “to step on.” Together these elements are technically a verb phrase, but in this case they act as a noun which is connected by the archaic associative particle が (ga) – filling the same role as modern の (no) – to the adjective 如し (gotoshi), “like,” “as,” in sentence-final form. Keep in mind that although the final element is in sentence-final form, this kotowaza doesn’t really function on its own; it needs context, and will generally be part of a longer sentence.
This phrase is remarkably similar to an English idiom, “[skating] on thin ice.” In fact, my policy of avoiding back-translations of phrases borrowed from English in the first place nearly led to hakuhyou wo fumu getting skipped… but it’s not from English at all! The origin of this one seems to be in the 2600-year-old Chinese Classic of Poetry (詩経), specifically the 74 “Lesser Court Hymns” section (小雅), written in the 9th to 8th centuries BCE. There, the phrase is coupled with another example of danger – the intense fear of gazing into a deep abyss.
This kotowaza can be shortened to 薄氷を履む.
(“Fui ni oogata torakku ni kakomarete, Sumisu san ga hakuhyou wo fumu ga gotoku kyoufu wo oboete ugokenaku natte shimaimashita.”)
[“Suddenly surrounded by large trucks, Smith was filled with fear akin to walking on thin ice, and became paralyzed.”]