Several parts combine into one character.

Compound ideograph with a poetic encoding: 十 (ten) on top + 口 (mouth) below. The story: "what has passed through ten mouths" — knowledge or news that has been retold so many times across generations that it qualifies as ancient. Ten generations of oral transmission = old. Identical across 繁體 / 新字体 / 简体.

This etymology rewards memorization because the character is a direct compositional check: when you see 十 + 口 = 古, you know the meaning is "ancient / old". Pairs with 新 (new — next entry) as one of the fundamental temporal oppositions.

Mandarin: gǔ, dipping 3rd tone. 古代 (gǔdài, ancient times), 古老 (gǔlǎo, ancient / time-honored), 古典 (gǔdiǎn, classical), 古迹 (gǔjī, historic site), 考古 (kǎogǔ, archaeology — "investigating the ancient"), 古玩 (gǔwán, antique). Pairs with 现代 (xiàndài, modern era) and 古代 in historiographic vocabulary.

Japanese: on-reading コ (ko) — 古代 (kodai, ancient era), 古典 (koten, classics), 中古 (chūko, second-hand / used — important for online shopping vocabulary). The Japanese 中古車 (chūkosha) market — used cars — is enormous, and the term means "middle-old car". Kun-reading ふる.い (furu.i) is the everyday adjective — 古い (furui, old / used / outdated), 古本 (furuhon, used book — note rendaku voicing), 古着 (furugi, used clothing).

古い vs 老い (oi): both can mean "old" in English, but 古 is for things (古い建物 = old building) while 老 is for people (老人 = elderly person). Japanese learners must keep these separate; calling a person 古い (furui) is grammatical but rude — implying they're obsolete.

Memory aid: ten mouths telling the same story — ancient by repetition.