hook, line and sinker

hook, line and sinker (сompletely; the whole thing) — целиком и полностью

When someone says: We bought it hook, line, and sinker,: they mean something like We believed the whole (false) story. — купились (на эту лажу); проглотили это, не поперхнувшись; попались на удочку, заглотили наживку; безоговорочно поверили; ≈ нас развели по полной программе (т.е. заставили поверить)

This expression is used with such verbs as buy, accept, fall for, etc. It has to do with being deceived.

fall for/believe /buy sth hook, line and sinker (to completely believe something that someone tells you that is not true) — безоговорочно поверить, купиться (на ложь)

Example 1: What an idiot! He believed the whole story hook, line, and sinker! (LDOCE)

Example 2: The people seem to have swallowed the government's promises hook, line, and sinker.
(LDOCE)

Example 3: She told him she needed the money for her baby and he fell for it hook, line, and sinker. (CALD))

Related Vocabulary
[попасться на удочку]
see also
[lock, stock, and barrel]
[stock and store]

with both feet - целиком, полностью