tödlich
deadly
Leading to death or causing death.
Der Sturz aus dieser Höhe war für ihn tödlich.
The fall from this height was deadly for him.

deadly
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deadly
Leading to death or causing death.
Der Sturz aus dieser Höhe war für ihn tödlich.
The fall from this height was deadly for him.

The adjective 'tödlich' (deadly) is derived directly from the noun 'der Tod' (death). The ending '-lich' means something like 'in the manner of' or 'belonging to'. Literally, it describes something that belongs to death or is like death.
Einige Pilzarten enthalten ein tödliches Gift.
Some types of mushrooms contain a deadly poison.
'Tödlich' (deadly) can literally mean 'leading to death' or figuratively serve as an intensifier meaning 'extremely'. The context makes the difference: if it refers to real dangers such as accidents or illnesses, it is literal. If it intensifies a feeling such as boredom or being offended, it is figurative.
Der Biss dieser Schlange ist für Menschen meist nicht tödlich.
The bite of this snake is usually not deadly for humans.
In colloquial language, the combination 'tödlich langweilig' (deadly boring) is a very common and fixed expression used to express extreme boredom. Here, 'tödlich' functions as an intensifying adverb.
Die Party war tödlich langweilig, deshalb bin ich früh gegangen.
The party was deadly boring, so I left early.
As an intensifier for feelings, 'tödlich' (deeply/extremely) is often used with hurt feelings or insults. Typical expressions are 'tödlich beleidigt' (deeply offended) or 'tödlich gekränkt' (deeply hurt), and they mean 'profoundly hurt'.
Er war tödlich beleidigt, weil er nicht zur Feier eingeladen wurde.
He was deeply offended because he was not invited to the celebration.
extreme
'extrem' and 'tödlich' can both express a very strong effect. 'extrem' is more general and more neutral: it can intensify many qualities, for example 'extrem kalt' (extremely cold) or 'extrem schwierig' (extremely difficult). 'tödlich' is much narrower as an intensifier. It is often used figuratively for very negative things, especially in fixed expressions like 'tödlich langweilig' (deadly boring) or 'tödlich beleidigt' (deeply offended). Literally, however, 'tödlich' means that something leads to death. For that reason, you should not use it everywhere as a replacement for 'extrem'.

Der Vortrag war so langsam, dass er für viele im Raum tödlich langweilig war.
The presentation was so slow that many people in the room found it deadly boring.
Die Arbeit im Lager war im Sommer extrem anstrengend.
Working in the warehouse was extremely exhausting in the summer.