1
general::
In a direct, forthright manner, as in I'll tell you, straight from the shoulder, that you'll have to do better or they'll fire you. This expression comes from boxing, where it describes a blow delivered with full force. Its figurative use dates from the late 1800s.
American Heritage Idioms
2
general::
Phrase(s): straight from the shoulder
Fig. very direct, without attenuation or embellishment. • Okay, I’ll give it to you straight from the shoulder. • Right straight from the shoulder: clean out your desk; you’re through.
McGrawhill's American Idioms And Phrasal Verbs