Guru
Etymology
From Hindi and Sanskrit guru (“teacher, spiritual guide”), whose literal sense is “heavy, weighty” — from the same ancient root as Latin gravis. The teacher is the “weighty one,” grave and worthy of respect. English borrowed the spiritual sense and then stretched it to any trusted expert.
noun · /GOO-roo/
A revered teacher or authority whose guidance others follow closely.
What it names
The trusted master in some domain — spiritual or worldly — whose word carries weight beyond ordinary expertise.
Example
He fashioned himself a productivity guru, though his own inbox held thousands of unread emails.
Related idea
A mentor guides you personally over time; guru implies revered, almost spiritual authority — often with a following attached.