abstruse

As adjectives the difference between abstruse and undefined

is that abstruse is (obsolete) concealed or hidden out of the way; secret while undefined is lacking a definition or value.

As adjectives the difference between abstruse and awkward

is that abstruse is (obsolete) concealed or hidden out of the way; secret while awkward is lacking dexterity in the use of the hands, or of instruments.

As an adverb awkward is

(obsolete) in a backwards direction.

As adjectives the difference between insoluble and abstruse

is that insoluble is that cannot be dissolved while abstruse is concealed or hidden out of the way; secret.

As adjectives the difference between abstruse and humble

is that abstruse is concealed or hidden out of the way; secret while humble is near the ground; not high or lofty; not pretentious or magnificent; unpretending; unassuming; as, a humble cottage.

As a verb humble is

to bring low; to reduce the power, independence, or exaltation of; to lower; to abase; to humiliate.

As adjectives the difference between impractical and abstruse

is that impractical is not practical; impracticable while abstruse is concealed or hidden out of the way; secret.

As an adjective abstruse

is concealed or hidden out of the way; secret.

As a noun opacity is

the state or quality of being opaque, not allowing light to pass through.

As adjectives the difference between abstruse and unnoticed

is that abstruse is concealed or hidden out of the way; secret while unnoticed is not noticed.

As adjectives the difference between hypothetical and abstruse

is that hypothetical is based upon a hypothesis; conjectural while abstruse is concealed or hidden out of the way; secret.

As a noun hypothetical

is a hypothetical situation or proposition.

As adjectives the difference between shadowy and abstruse

is that shadowy is in shadow; darkened by shadows while abstruse is concealed or hidden out of the way; secret.

As an adjective abstruse

is concealed or hidden out of the way; secret.

As a noun enigma is

something or someone puzzling, mysterious or inexplicable.

As a proper noun Enigma is

a German device used during World War II to encode strategic messages.