عُسٌّ
1.
A [drinking-cup or bowl, of the kind called] قَدَح: (TA:) or a large
قَدَح, (S, A, Mgh, O, L, Msb, K,) from which two or three or more [men] may satisfy their thirst; (L, TA;) larger than the
غُمَر; (L, voce رِفْدٌ:) though this is greater in height; (TA;) and larger than the
قَعْب; (IAar, in TA, voce قَعْبٌ;) but not so large as the
رِفْد: (S, O:) plural عِسَاسٌ (S, A, Mgh, O, Msb, K) and عِسَسَةٌ (TA) and [plural of pauc.] أَعْسَاسٌ. (IAth, Msb.)
2.
Hence, one says, هُوَ
لَكَ عَلَى ظَهْرِ العُسِّ, meaning (assumed tropical:) It is apparent, manifest, or conspicuous, to thee. (O in article ظهر.)