wassail is from Middle English wæs hæil, a calque of Old Norse ves heill "be healthy". Apparently the expression was used as a toast by the Danes in England, and it spread to the English. One said wæs hæil when drinking someone's health, or when presenting a cup of wine to someone. The reply was drinc hæil "drink healthy".
heill and hæil are cognate with hale, whole and healthy and are from Proto-Indo-European *kailo-.
wæs, ves are from Proto-Indo-European *h₂wes- "to live, dwell, pass the night", which gives us was and were. The suffixed form *h₂wes-eno- became Old Persian vahanam "house" according to Pokorny and the AHD. Also according to the AHD, this became Persian دیوان dīvān, which could refer a collection of poems, account book, or royal court. Borrowed into English as divan, it originally referred to "An Oriental council of state", then "A room having one side entirely open towards a court", then "a long seat consisting of a continued step, bench, or raised part of the floor, against the wall of a room, which may be furnished with cushions, so as to form a kind of sofa or couch" (OED). That's what I think when I think divan: a sort of long chair-type thing with cushions.
The word shows up in French as douane "customs".