Imaginable means able to be pictured in the mind. If you can conceive of it, it is imaginable.
The word rarely stands alone, and it usually props up a superlative: every colour imaginable, the worst outcome imaginable, in every way imaginable. Placed after the noun like that, it stretches the claim as far as it will go. The negative, unimaginable, is commoner still and is used for horror and scale alike. Do not confuse it with imaginary, which means not real at all, and is a completely different word.
1adjectiveimaginableAble to be pictured or conceived of.
“They stocked every kind of nail imaginable.”
Latin
imāginābilis
Middle English
ymaginable
English, today
imaginable
From Middle English ymaginable, from Latin imāginābilis; equivalent to imagine + -able.
Etymology from Wiktionary, CC BY-SA