really, awfully, terribly,
 [adverb] [‘hart-sti-ku’]  Iconspeaker_3

is one of those words that start to sound really strange when you
repeat it a few times. But it is in fact a strange word that has
nothing to do with its usage today. Around 1600 the expression
"hartstikke dood" was used when somebody had died as a result of a stab
in the heart. "Hartstikke" was derived from "hertsteken": to pierce the
heart with iron.
At one point, when the origin of the expression was not well-known
anymore, one started to use "hartstikke" as an adverb to intensify.
This lead to the current usage of "hartstikke".

Note that, probably due to its origin, you can only use "hartstikke"
as an adverb and not as an adjective, contrary to the suggested
translations above.

"Hartstikke" is a very common adverb, try to use it!

– "Hoe was je vakantie?" – "Heel erg ontspannend, maar het was hartstikke warm!"
("How were your holidays?" – "Very relaxing, but it was terribly warm!")

– "Is jouw baan niet enorm saai?" – "Nee, juist niet, mijn baan is hartstikke leuk!"
("Isn’t your job terribly boring?" – "No, on the contrary, my job is absolutely wonderful!")

– "We hebben hartstikke lekker gegeten bij restaurant Rozemarijn, maar de bediening was vrij slecht."
food at restaurant Rozemarijn was really very tasty, but the service
was pretty bad." Literally: "We have eaten very tastefully…")

– "Mag ik jou complimenteren met het resultaat? Hartstikke goed gedaan!" – "Nou, hartstikke bedankt dat je dat zegt!"
("May I compliment you on the results? Very well done!" – "Well, thank you very much indeed for telling me this!")

Related words:
– "Enorm": enormous(ly).
– "Verschrikkelijk": terrible/y.
– "Fantastisch": fantastic.
– "Uitermate": very.