Peace be upon you
Here is the meaning of the word from Edward Lanes Lexicon. I copied the below from the site
http://www.studyquran.co.uk/PRLonline.htm, which I recommend to use when you are in doubt about a meaning.
To think, suppose, doubt, assume, deem, believe, know, imagine, suspect, conjunctive, be sure of something in view of one's observations. As a general rule often this verb is succeeded by 'anna or 'an, that means to be sure about.
Zanna (prf. 3rd. p.m. sing, assim.): He thought, imagined, deemed, assumed, believed, conjectured, suspected; He was sure.
Zanantu (prf. 1st, p.m. sing.): I was sure.
Zannaa (prf. 3rd. p.m. dual.): The twain thought.
Zannuu (prf. 3rd. p.m. plu.): They imagined.
Zanantum (prf. 2nd p.m. plu.): Ye thought.
Zananna (prf. 1st. p. plu.): We thought.
Yazunnu (imp. 3rd. p.m. sing.): He thinks.
Tazunnu (imp. 3rd. p. f. sing.): She thinks.
Azunnu (imp. 1st. P. sing.): I think.
Yazunnuuna (imp. 3rd, p. in. plu.): They know, believe, conjuncture.
Tazunnuuna (imp. 2nd. p.m. plu.): You entertained wrong thoughts.
Nazunnu (imp. 1st. p. plu.): We deem.
Zannun/Zannan (nom./acc.n.): Thinking; Conjecture.
Zunuuna (n. plu.): Diverse thoughts.
Zaanniina (act. pic. n. plu.): Entertainers of evil thought.
يظنون yathunnoona - They know/believe/conjecture [2:46, 78, 249; 3:154; 45:24]
ظنا thannaan - Thinking, conjecture, idea [2:230; 10:36; 45:32]
ظن thannu/a - He thought, imagined, deemed, believed, assumed, conjectured, suspected; He was sure [3:154; 10:60; 12:42; 24:12; 38:27; 48:6, 12; 84:14]
الظن thanni/a - [4:157; 6:116, 148; 10:36, 66; 49:12 (2x); 53:23; 28 (2x)]
لنظنك thunnuka - Believe, think [7:66]
وظنوا thannoo - They imagined [7:171; 9:118; 10:22; 12:110; 28:39; 41:48; 59:2]
ظننتم thanantum - You thought [41:22, 23; 48:12; 59:2; 72:7]
ظنوا thannoo - They thought [72:7]
وظن thanna - Conclude [10:24; 38:24; 75:28]
نظنكم nathunnu - We think [11:27]
وتظنون thunnoona - Diverse thoughts, You thought [17:52; 33:10]
لاظنك thunnu - I think [17:101, 102]
فظنوا thannoo - They imagined [18:53]
فظن thanna - He imagined [21:87]
يظن yathunnu - He thinks [22:15; 83:4]
نظنك nathunnu - We deem [26:186]
لاظنه athunnu - I think [28:38; 40:37]
ظنه thanna - His idea [34:20]
ظنكم thannu - [37:87; 41:23]
نظن nathunnu - We deem [45:32]
الظانين thanneena - Entertainers of evil thought [48:6]
ظننت thanantu - I was sure [69:20]
ظننا thanannaan - We thought [72:5, 12]
تظن Tathunnu - She thinks [75:25]
LL, V5, pages: 209
http://www.studyquran.org/LaneLexicon/Volume5/00000209.pdf, 210
http://www.studyquran.org/LaneLexicon/Volume5/00000210.pdf, 211
http://www.studyquran.org/LaneLexicon/Volume5/00000211.pdfYou can check from other lexicons from here as well:
http://ejtaal.net/aa/#q=Znn