get the dope on

Webster's Dictionary (GCIDE)

1.Any thick liquid or pasty preparation, as of opium for medicinal purposes, of grease for a lubricant, etc. [Webster 1913 Suppl.]

2.Any preparation, as of opium, used to stupefy or, in the case of a race horse, to stimulate. [Slang or Cant] [Webster 1913 Suppl.]

3.An absorbent material; esp., in high explosives, the sawdust, infusorial earth, mica, etc., mixed with nitroglycerin to make a damp powder (dynamite, etc.) less dangerous to transport, and ordinarily explosive only by suitable fulminating caps. [Webster 1913 Suppl.]

4.Information concerning the previous performances of race horses, or other facts concerning them which may be of assistance in judging of their chances of winning future races; similar information concerning other sports; by extension, any information not generally known, especially when coming from an inside source; as, the inside dope. [Sporting Slang] [Webster 1913 Suppl. +PJC]

5.an ignorant or stupid person. [Colloq.] [PJC]

get the dope on learn the true story; get the inside information.