母衣月

• poro be big
• + tuki sake cup

Proponent: 既存の地名解 (the current explanations)

tuki is a cup for sake, so the readings that take this route see the mountain as an upturned cup, or Suttsu Bay as a cup filled to the brim.

• poro be big
• + tuk push out, grow out, extend
• + i place where

Proponent: zenibako_trek（小樽銭函地名歴史研究会） (zenibako_trek (Otaru Zenibako Place-Name and History Society))

tuk-i stands in the place-name dictionary as 'knoll', and the author takes it to carry the sense of tuk 'grow out, extend': a place that shoots up. The whole Suttsu peninsula is seen that way, and the highest point in it was called poro-tuk-i. Tsukikoshi, the name of the range, means the crossing over Mt. Horotsuki; the pass from Mikawa in Shimamaki over the Tsukikoshi moor down to Kuromatsunai is well made up today, and Matsuura's sketch shows the older route coming down towards Yaoi instead, which may have passed near the summit.