Arsis and thesis

In music and prosody, arsis and thesis (plural arses and theses) refer to the stronger and weaker parts of a musical measure or poetic foot. Arsis and thesis were the raising and lowering of the foot in beating of time or in marching or dancing. An ancient Greek writer, Bacchius, states: "What do we mean by arsis? When our foot is in the air, when we are about to take a step. And by thesis? When it is on the ground."

Accordingly, in music and in Greek scansion arsis is an unaccented note (upbeat). However, in discussions of Latin and modern poetry the word arsis is generally used to mean the stressed syllable of the foot, that is, the ictus.

Since the words are now used in the opposite of their original meanings, the authority on Greek metre Martin West recommends abandoning them and using substitutes such as ictus for the down beat when discussing ancient poetry.

In Latin (and Greek) dactylic hexameter, the strong part of a foot is the first syllable — always long — and the weak part is what comes after — two short syllables (dactyl: long—short—short) or one long syllable (spondee: long—long). Because Classical poetry was not based on stress, the arsis is often not stressed; only consistent length distinguishes it.

In English, poetry is based on stress, and therefore arsis and thesis refer to the accented and unaccented parts of a foot.

Ancient Greek ἄρσις ársis "lifting, removal, raising of foot in beating of time", from αἴρω aírō or ἀείρω aeírō "I lift". The i in aírō is a form of the present tense suffix y, which switched places with the r by metathesis.

...
Wikipedia