From Wiktionary, the free dictionary
From tank + side.
tankside (plural tanksides)
- The side of a tank (for water etc.)
1964 July, “Newsreel”, in Modern Railways, page 67, photo caption:An ex-LMS Class 3F 0-6-0T, No. 47482, emerges from Darlington works after overhaul with its number painted on the tankside in NER style.