I ask this question thinking of my native language, Italian, in which "guardare di traverso" has a negative meaning, implying the idea of a sly or threatening look. Is it the same in English, or, in itself, does it just mean "to look by turning one's eyes sideways"? Dictionaries don't help me much.
No. A "sideways glance" or "side-glance" can have a negative connotation, but not necessarily. In writing, it depends entirely on the context.
Physically, it has two meanings:
- looking at something without turning your head towards it
- looking away from something without turning your head away
Some examples without having a negative meaning are glancing over at something only briefly because you're not interested in it, glancing at someone quickly to be flirty, looking away from someone because you're confused, and so on.
The expression in English for a sly and threatening look would be "glare", like Vin Diesel here: https://giphy.com/gifs/thefastsaga-fast-and-furious-saga-7-IUxFvKwD3jXisqR5w7
There is an expression that has spiked in use in the past few years (though it was used as early as 1797) that carries exactly this connotation: "side-eye." It's often used in sentences as if it's a non-count noun: "She gave me some serious side-eye." The Merriam-Webster entry:
: a sidelong glance or gaze especially when expressing scorn, suspicion, disapproval, or veiled curiosity
// The guy who stole your heart as the class clown can seem like just a clown out of his original context, like when people are giving him side-eye for cracking lame jokes in the hostess line.
—often used with the
// … the singular focus on results washes away concerns about getting the side-eye from a colleague judging you for not being in your cubicle …
Although Webster reports that 1797 usage, this would generally be received as an informal and very contemporary usage today.