1
general::
1. Make plain, clarify, as in We asked her to spell out her objectives. [c. 1940] 2. Read slowly and laboriously, as in He was only six but he managed to spell out the instructions. [Early 1800s] 3. Puzzle out, manage to understand with some effort, as in It took years before anyone could spell out the inscriptions on the Rosetta Stone. [Late 1600s] All three usages transfer spell in the sense of "proceed letter by letter."
American Heritage Idioms
2
general::
Phrase(s): spell something out
1. Lit. to spell something (with letters). • I can’t understand your name. Can you spell it out? • Please spell out all the strange words so I can write them down correctly. 2. Fig. to give all the details of something. • I want you to understand this completely, so I’m going to spell it out very carefully. • The instruction book for my computer spells out everything very carefully.
McGrawhill's American Idioms And Phrasal Verbs