giftig
poisonous / venomous
Containing poison and therefore harmful to health or deadly; toxic.
Der Knollenblätterpilz ist einer der giftigsten Pilze in Europa.
The death cap mushroom is one of the most poisonous mushrooms in Europe.

poisonous / venomous
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poisonous / venomous
Containing poison and therefore harmful to health or deadly; toxic.
Der Knollenblätterpilz ist einer der giftigsten Pilze in Europa.
The death cap mushroom is one of the most poisonous mushrooms in Europe.

'Giftig' comes from 'das Gift' (poison) and means 'poisonous'. It is a well-known false friend of the English word 'gift' (Geschenk).
Vorsicht, diese Beeren sind giftig!
Careful, these berries are poisonous!
The adjective 'giftig' (poisonous) is derived directly from the noun 'das Gift' (poison). Anything that contains 'Gift' or acts like 'Gift' is 'giftig'.
In der Fabrik wird mit giftigen Chemikalien gearbeitet.
The factory works with toxic chemicals.
You can recognize the meaning of 'giftig' from the context. If it is about animals, plants, or chemistry, it means 'poisonous'. With people and communication, it means 'spiteful' or 'malicious'. In connection with colors, it describes a garish, intense shade.
Manche Schlangen sind extrem giftig.
Some snakes are extremely venomous.
In its literal meaning, 'giftig' is often used for living creatures or substances. Typical phrases are 'giftige Pilze' (poisonous mushrooms), 'giftige Schlangen' (venomous snakes), 'giftige Pflanzen' (poisonous plants), or 'giftige Dämpfe' (toxic fumes).
Beim Putzen sollte man das Fenster öffnen, um keine giftigen Dämpfe einzuatmen.
When cleaning, you should open the window so you don't inhale toxic fumes.
In the figurative sense, 'giftig' describes a negative, aggressive way of communicating. Common combinations are 'eine giftige Bemerkung' (a spiteful remark), 'ein giftiger Blick' (a venomous look), or 'ein giftiger Tonfall' (a venomous tone of voice).
Er warf ihr einen giftigen Blick zu.
He gave her a venomous look.
In this meaning, 'giftig' is almost always used as part of a compound adjective. The most common form is 'giftgrün' (garish/poison green), but 'giftgelb' (garish yellow) or 'giftblau' (garish blue) are also possible to describe an unnaturally bright color.
Sein neues Fahrrad ist giftgrün.
His new bike is garish green.
dangerous
'gefährlich' and 'giftig' both have to do with possible harm. 'gefährlich' is the more general word: a road, an animal, a situation, or a tool can be dangerous. 'giftig' is more specific and means that something contains poison and can therefore make someone sick or kill them. The words can be confused because poisonous things are also dangerous. But not everything dangerous is poisonous. If the harm comes from poison, you say 'giftig'.

Iss diese Beeren nicht, weil sie giftig sein können.
Do not eat these berries, because they may be poisonous.
Die kaputte Treppe ist gefährlich, deshalb benutzen wir sie nicht mehr.
The broken staircase is dangerous, so we do not use it anymore.