届

Outer 尸 radical — depicts a body bent at the waist, the 'reclining-figure' shape. As the indexing radical it groups 届 with other body-position chars: 居 (reside), 屋 (house), 屁 (fart), 层 (layer). Here it caps the inner phonetic, framing the character in the dwelling/term-of-residence sense (a session, a tenure) rather than literal corpse.

Inner 由 supplies the sound (yóu → jiè, with palatalization shift over centuries; faint trace remains). Tucked under 尸 it pictures a sprouting shoot or vessel with a stem — a fitting visual for 'a term that comes due': each session sprouts in turn, then yields to the next 届.