上～
うわ～ · uwa～ · JLPT N2 · noun
upper berth
uwa is ue (上, above) in compound form: e shifts to a, as in sake → saka-
Before another word, 上 ue becomes uwa-, the same e→a shift seen in 目 me → ma- and 酒 sake → saka-. Examples: 上着 uwagi (outer jacket = upper + wear) and 上履き uwabaki (indoor shoes = upper + footwear).
上 — horizontal line above a baseline
上 is an indicative character (指事): a short stroke placed above a longer reference line shows the concept of 'above' or 'up'. The earliest oracle-bone forms used two horizontal lines for this; the modern shape added a vertical stroke for clarity. The core meaning of 'upper, above, to ascend' has stayed stable for millennia.