KanjiJLPT N1
Third Class

third class, 3rd, 3rd calendar sign

Mnemonic

One thick line seals a crate shut. You pry it open and look inside — nothing but Third Class goods. Cheap, mediocre stuff crammed inside one shabby container. Not even worth the shipping costs.
Additional thoughts
Picture the horizontal stroke of One as a lid. Inside the box beneath it, everything is disappointingly average — C-grade, third-tier junk. The shape of the kanji itself looks like a sealed box.
Quick recall
One lid over a box; peek inside to find only Third Class junk.

Details

The keyword for 丙 is third class. In traditional East Asian ranking systems, 丙 represents the third position in the celestial stems (天干), following 甲 (first) and 乙 (second). It is commonly used to denote a third-tier grade or ranking, similar to receiving a "C" in Western grading systems. By extension, it can imply something of mediocre or below-average quality.
On'yomi
へい
Kun'yomi
ひのえ

Used in kanji