Sidelong means directed to one side rather than straight ahead. A sidelong glance, look sidelong, a sidelong remark.
It nearly always describes a glance, and the sideways look carries suspicion or amusement that a direct one would have to admit to. The -long ending is not the word long — it comes from an old ending meaning in the manner of, as in headlong.
1adjectivesidelongDirected to one side rather than straight ahead.
“She gave me a sidelong look when I mentioned the missing money.”
sidelong is a direction of looking.
From side plus an old ending -ling meaning in the manner of, later reshaped to -long. Headlong is built the same way.
Middle English
sidlyng
English, today
sidelong
Built from
1520s, from Middle English sidlyng (early 14th century), from side + -lyng (“(frequentative)”) (modern English -le (“(frequentative)”)). Reanalyzed as side + long.
Etymology from Wiktionary, CC BY-SA