
überhaupt
at all
Used in negative sentences and questions to generalize or intensify a statement; often in the sense of 'gar' (not at all) or 'at all'.
Ich habe heute überhaupt keine Zeit für ein Treffen.
I have no time at all for a meeting today.
at all / generally
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at all
Used in negative sentences and questions to generalize or intensify a statement; often in the sense of 'gar' (not at all) or 'at all'.
Ich habe heute überhaupt keine Zeit für ein Treffen.
I have no time at all for a meeting today.

generally
Means 'in general' or 'basically' and introduces a general statement.
Ich mag Sport und Fußball überhaupt am liebsten.
I like sports, and football is generally my favorite.

actually
Used in questions to express surprise, doubt, or impatience.
Wie konntest du das überhaupt vergessen?
How could you actually forget that?

if at all
A fixed expression that marks a possibility as very small or unlikely. It means 'if it even happens at all'.
Wir sehen uns selten, wenn überhaupt nur einmal im Jahr.
We rarely see each other, if at all only once a year.
'Überhaupt' is made up of 'über' (over) and 'Haupt' (head). The literal idea is 'looked over the head', which makes the meaning 'by and large' or 'generally' understandable.
Wir müssen überhaupt erst einmal klären, was das Ziel ist.
First of all, we need to clarify what the goal is.
In negative sentences, 'überhaupt' intensifies the negation. It often comes directly before the negated part, in the combinations 'überhaupt nicht' (not at all) or 'überhaupt kein/e' (no ... at all).
Ich kann mich überhaupt nicht an seinen Namen erinnern.
I can't remember his name at all.
In a question, 'überhaupt' often expresses doubt, surprise, or impatience. It calls the basis of the question or a prerequisite into question.
Weißt du überhaupt, wie spät es ist?
Do you even know what time it is?
'Überhaupt' can introduce a general or fundamental statement. It then means something like 'in general' or 'in principle'.
Die Lage auf dem Wohnungsmarkt ist überhaupt sehr angespannt.
The situation on the housing market is generally very tense.
The expression 'wenn überhaupt' (if at all) strongly limits a statement and expresses that something is very rare or unlikely. It means 'if it happens at all'.
Er meldet sich nur selten, und wenn überhaupt, dann nur per SMS.
He only rarely gets in touch, and if at all, then only by text message.
because
Denn and überhaupt can express surprise or impatience in questions. Denn mainly colors the question through the immediate conversational context, while überhaupt also more strongly questions the underlying assumption or possibility. Use denn for a normal follow-up question and überhaupt when you doubt that something is actually the case.

Du willst mitkommen, willst du überhaupt losfahren?
You want to come along, but are you even planning to leave?
Du willst mitkommen, wann fährst du denn los?
You want to come along, so when are you leaving?