solus adj.

the Latin word for "alone" (see sole (adj.)), used in English by 1590s, originally and used chiefly in stage directions. The word in this form is masculine; the fem. is sola, but in stage directions solus typically still serves for both.

It also is found in phrases such as solus cum sola (the name of an old popular song) "alone with an unchaperoned woman," and solus cum solo "all on one's own," both literally meaning "alone with alone."