lost cause

get lost

To exit from the scene.

look as if one had lost a shilling and found sixpence

To look very disappointed.

look as if one has lost a shilling and found sixpence

To look annoyed or displeased.

lost errand

Synonym of fool's errand.

lost in the shuffle

Failing to stand out or receive attention as a result of a larger-scale change or transformation.

lost in translation

Unable to be understood due to having been poorly translated.

lost on

Not understood or appreciated by (someone).

lost to the world

Of a thing, event, etc., no longer used or available; unknown to or forgotten by everyone.