lit. billy goat's horns caught in the fence (idiom from Book of Changes 易经); impossible to advance or to retreat; in an impossible situation; without any way out of a dilemma; trapped
羝羊：公羊。藩：篱笆。公羊撞篱笆，角被缠住了。形容进退两难。
主谓式；作谓语、宾语；比喻进退两难
羝羊：公羊；触：抵撞；藩：篱笆。公羊的角缠在篱笆上，进退不得。比喻进退两难。
《周易·大壮》：“羝羊触藩，羸其角。不能退，不能遂。”
纣王沉吟不语，心下煎熬，似羝羊触藩，进退两难。◎明·许仲琳《封神演义》第七回