IPA: /ˈtɒpˌsaɪdə/
KK: /ˈtɑːpˌsaɪdər/
A person who holds the highest position or authority in a group or organization.
As the CEO, she is the topsider of the company, making all the major decisions.
Topsider is formed from "top" (from Old English *toppa*, meaning the highest point) and "-sider" (from Old English *sīde*, meaning side). The word refers to something that is positioned at the highest point or on the upper side.
Think of the 'top' as the highest point and 'sider' as a side — a topsider is something that is on the upper side.
No commonly confused words.