altmodisch
old-fashioned
No longer in line with current fashion, current taste, or modern technology; not modern.
Meine Großmutter hat ein altmodisches Telefon mit Wählscheibe.
My grandmother has an old-fashioned telephone with a rotary dial.

old-fashioned
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old-fashioned
No longer in line with current fashion, current taste, or modern technology; not modern.
Meine Großmutter hat ein altmodisches Telefon mit Wählscheibe.
My grandmother has an old-fashioned telephone with a rotary dial.

The word is made up of 'alt' (old) and 'modisch' (fashionable). So it literally describes something that is in an old fashion.
Er trägt einen für seinen Geschmack etwas zu altmodischen Anzug.
He is wearing a suit that is a little too old-fashioned for his taste.
'Altmodisch' (old-fashioned) can have a negative meaning (outdated, not modern), but also a positive one (charming, nostalgic, retro). The tone and context determine the exact nuance.
Ihre Methoden sind hoffnungslos altmodisch und ineffizient.
Her methods are hopelessly old-fashioned and inefficient.
If the word refers to concrete things like clothing, furniture, or technology, the meaning is 'not modern'. If it refers to abstract things like ideas, values, or behavior, it means 'outdated views'.
Sein altmodisches Auto ist ein echter Hingucker.
His old-fashioned car is a real eye-catcher.
yesterday
'von gestern sein' (to be outdated) and 'altmodisch' both mean that something does not seem modern. 'altmodisch' is a normal adjective and can describe clothing, furniture, technology, taste, or also opinions. 'Von gestern sein' is an idiom and sounds more like a judgment: something no longer fits today's world. 'Altmodisch' can sometimes also sound neutral or even friendly, for example when talking about a style. 'Von gestern' is usually more critical.

Sie trägt gern altmodische Kleider, weil ihr der Stil gefällt.
She likes wearing old-fashioned dresses because she likes the style.
Diese Einstellung ist nicht mehr passend; sie ist von gestern.
This attitude is no longer appropriate; it is outdated.