the.com/straightener

means A heated clamping tool that presses hair flat between hot plates to smooth out curls, waves, or frizz.

from A straightforward agent-noun: 'straighten' (from 'straight,' tracing back to Old English 'streht,' the past participle of 'streccan,' meaning to stretch) plus the '-er' that turns a verb into 'the thing that does it.' The hair-tool sense is modern, arriving alongside electric heating elements in the 20th century — before that, people used actual flat irons, heated on stoves, which is why the tool is still often called a 'flat iron' today.