The Free On-line Dictionary of Computing (19 January 2023):

IMHO
IMAO
IMNSHO
IMO

    (From SF fandom via Usenet) In My Humble Opinion.
   Also seen in variant forms such as IMO, IMNSHO (In My
   Not-So-Humble Opinion) and IMAO (In My Arrogant Opinion).

   [Jargon File]

   (1998-09-24)


The Jargon File (version 4.4.7, 29 Dec 2003):

IMHO
 //, abbrev.

    [from SF fandom via Usenet; abbreviation for ‘In My Humble Opinion’] “IMHO,
    mixed-case C names should be avoided, as mistyping something in the wrong
    case can cause hard-to-detect errors — and they look too Pascalish anyhow.”
    Also seen in variant forms such as IMNSHO (In My Not-So-Humble Opinion) and
    IMAO (In My Arrogant Opinion).