know-not-what
/ˌnəʊnɒtˈwɒt/Now rare
noun
An indescribable thing; a ‘whatchamacallit’. Also with the: that which is indescribable or unknown.
Origin
Mid 17th century; earliest use found in John Suckling (d.?1641), poet. From know + not + what adj.1 adverb.