1. somewhat dim; slightly dark or not bright
Usage: informal; chiefly British
2. somewhat dull in intellect; not very intelligent
Usage: informal; derogatory

Examples

• The dimmish light in the old library made it hard to read the small print.
• He gave a dimmish response to the straightforward question.
• The room felt dimmish even with all the lamps turned on.
• She's not dimmish—she just wasn't paying attention.
• The dimmish glow of the streetlamp barely reached the corner.
• His dimmish understanding of the subject became apparent during the discussion.