on top of that
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on top of that
Expresses that something is added to something already mentioned; also, in addition, furthermore.
Das Essen war kalt und der Service obendrein noch unfreundlich.
The food was cold, and on top of that the service was unfriendly.
The adverb is made up of 'oben' (above/on top) and 'drein' (an old form of 'darin', meaning in it). The literal meaning is 'on top of it,' which explains the figurative meaning 'in addition' well.
Er hat die Prüfung nicht bestanden und obendrein sein Lehrbuch verloren.
He didn't pass the exam and on top of that lost his textbook.
'Obendrein' (on top of that) can stand at the beginning of a sentence (position 1). In this case, the verb follows directly after it. It can also stand in the middle of the sentence to emphasize additional information.
Die Wanderung war sehr lang. Obendrein fing es stark an zu regnen.
The hike was very long. On top of that, it started raining heavily.
'Obendrein' (on top of that) is often used to highlight additional, usually negative or unexpected information. It gives the statement an emphatic, sometimes slightly complaining tone.
Ich habe meinen Zug verpasst und musste obendrein eine Stunde auf den nächsten warten.
I missed my train and on top of that had to wait an hour for the next one.
to that / additionally
'dazu' and 'obendrein' can both express that something additional is being added. 'dazu' is neutral and very general: you can add something to something else or take something extra with an order. 'obendrein' emphasizes more strongly that something is added on top, often with the feeling of 'as well' or 'in addition'. It sounds somewhat more emphatic and less everyday than 'dazu'. If you simply mean an addition, 'dazu' usually fits. If you want to especially emphasize the extra, 'obendrein' fits better.
Maria musste lange arbeiten und obendrein noch den Bericht fertig schreiben.
Maria had to work late and on top of that still had to finish the report.
Zum Salat nehme ich noch ein Stück Brot dazu.
I'll have a piece of bread with the salad.