massiv
massive
In a figurative sense: very strong, intense, extensive, or serious.
Der Plan stieß auf massiven Widerstand in der Bevölkerung.
The plan met with massive resistance among the population.

massive / solid
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massive
In a figurative sense: very strong, intense, extensive, or serious.
Der Plan stieß auf massiven Widerstand in der Bevölkerung.
The plan met with massive resistance among the population.

The meaning of 'massiv' (massive/solid) depends strongly on the noun. With abstract nouns like 'Kritik' (criticism), it means 'heftig' (intense/severe). With materials like 'Holz' (wood), it means 'solide/nicht hohl' (solid/not hollow).
Der Minister steht wegen der Affäre unter massivem Druck.
The minister is under massive pressure because of the affair.
In a figurative sense, 'massiv' (massive) describes a large extent or a high intensity. Typical collocations are 'massive Probleme' (massive problems), 'massive Kritik' (massive criticism), or 'massive Auswirkungen' (massive effects).
Die Bauarbeiten verursachen massive Lärmbelästigung.
The construction work is causing massive noise pollution.
When 'massiv' (solid) describes materials, it means that something consists entirely of that material and is not hollow. It is often used for high-quality or heavy objects.
Der Ring ist aus massivem Gold und sehr wertvoll.
The ring is made of solid gold and is very valuable.
The meaning 'großartig' (great) or 'toll' (awesome) is purely youth slang and very informal. In formal or neutral conversations, this usage should definitely be avoided.
Das Konzert gestern war einfach massiv!
The concert yesterday was simply awesome!
heavy / difficult
'schwer' and 'massiv' can both have to do with weight, size, or great intensity. 'schwer' directly says that something weighs a lot or that something is serious and grave. 'massiv' more often describes something as very solid, bulky, or very strong. 'Ein massiver Tisch' (a solid / massive table) looks large and sturdy; it may also be heavy, but 'massiv' says more about its construction or impression. In a figurative sense, people say things like 'massive Kritik' (massive criticism) or 'massive Probleme' (major problems). Then it means: very strong or extensive. 'schwer' is more neutral and more common when talking about weight or degree of severity.

Im Bericht steht, dass die Firma massive Probleme mit der Lieferung hatte.
The report says that the company had major problems with the delivery.
Der alte Schrank ist so schwer, dass wir ihn zu dritt tragen müssen.
The old cabinet is so heavy that three of us have to carry it.