His judgment was purely subjective conjecture, with no facts to back it up.
Tips
usage
臆 originally meant the chest, then extended to inner feelings and, most commonly today, baseless subjective guessing: 臆测 (to conjecture) and 臆断 (an arbitrary judgment).
Right side 意 (thought, idea) gives the sound yì exactly. Its meaning also feeds the modern sense: the chest as the seat of one's private thoughts and guesses.