As plain as a pikestaff and Unprepossessing

Unprepossessing and as plain as a pikestaff are semantically related. In some cases you can use "Unprepossessing" instead an adjective phrase "As plain as a pikestaff".

Unprepossessing adjective - Creating an unfavorable or neutral first impression.

As plain as a pikestaff and unprepossessing are semantically related. You can use "As plain as a pikestaff" instead an adjective "Unprepossessing".