étsus comes from the pronoun ecínos ‘that one’, which becomes étsnus and ultimately étsus with tsitacism, stress shift to the left, deletion of the unstressed /i/ and simplification of the consonant cluster (Ralli 2017).
E.g. étsus én íni mi ta sustá t.
etsínos ðen íne me ta sostá tu.
‘That man is insane’.