hair straighteners (pair of metal tongs with heated ceramic plates used for straightening hair)

a chapel de fer helmet: a kettle hat

Alternative form of chapeau de fer: a chapel de fer (helmet)

plural of fil de fer barbelé

barbed wire

(simile, informal) to firmly believe in, to be firmly convinced of [+ à (something)]

(transitive) to brand (to burn the flesh with a red-hot iron, as a marker) / (transitive, figuratively) to leave a deep impression on, to mark for life

(simile) to be a very poor swimmer

Alternative form of battre le fer pendant qu'il est chaud

(figuratively) to strike while the iron is hot, to make hay while the sun shines